The presence and nature of the accreting source in acc-SySt systems,
either a WD or a NS, is best defined by satellite observations performed in
the ultraviolet and X–rays, where the peak of the emissivity for
accretion-induced processes is located (e.g. [1]}
and references therein, and also [2]} for a recent review).
The X–ray/UV properties of the acc-SySt prototype SU Lyn have been
investigated by Mk16, [3]}, and [4]}
based on observations with Swift, NuSTAR and ASTROSAT
satellites. They found SU Lyn to show (i) a large (by a factor of a few
hundred in flux) UV excess with respect to a `normal' red giant; (ii)
long-term (months to years) hard X–ray variations by an order of magnitude
between low- and high-level states in the 15–35 keV hard X–ray emission;
and (iii) fast UV variability during the high X–ray states (flickering).
In the latter ones, the average X–ray flux was \(\sim \) 10\(^{-11}\)  erg
cm\(^{-2}\)  s\(^{-1}\)  in the 0.3–50 keV band, with the spectrum modelled with
a thermal plasma of temperature \(kT \sim 20\)  keV, plus a fluorescent iron
emission at 6.4 keV, absorbed by a hydrogen column N\(_{\rm H}\)  as large as
\(\sim \) 3\(\times \) 10\(^{22}\)  erg cm\(^{-2}\) . This absorption has however little
effect on the UV excess observed from this source. At a distance of 650 pc,
this corresponds to unabsorbed X-ray luminosities of up to \(\sim \) 10\(^{33}\) 
erg s\(^{-1}\)  in the high state, and a factor of 10 larger in the UV. The
passage from low- to high-level states in the UV and X-ray emission is
attributed to large excursions in the transfer and accretion rates.
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